This invention relates to novel solder compositions or solder creams and is particularly concerned with the provision of solder compositions especially adapted for soldering components in densely packed microelectronic circuits and providing dense solder joints, no solder balls and substantially reduced residue.
A solder cream is a material that is capable of being applied to a substrate or surface in a specific pattern using screening or analogous methods which can subsequently undergo fusing to provide an electrical joint or interface commonly described as a solder joint. The solder cream generally consists of metallic particles of various alloys, such as tin-lead, tin-lead-silver, etc., contained in a vehicle including a flux, such as wood rosin or derivatives thereof.
Currently, microelectronic circuits are made by surface-mounting chip carriers and other components on ceramic and plastic substrates by vapor phase soldering. Circuits similar to those used on printed circuit boards are produced on the substrates. In certain instances, the circuit is located on one side of the substrate, with pads being provided in certain areas, other parts of the circuit being covered with an insulating material.
The solder cream having a paste-like consistency is applied, for example by means of a silk screen, to the pads on the circuit board. Thereafter, the electronic components are carefully positioned with their peripheral contacts on the solder cream-coated pads. When all of the components are thus in place, the board, with such components temporarily positioned and retained thereon by the solder cream, can be placed in a vapor reflow system and subjected to vapor-phase condensation soldering at a sufficiently high temperature to cause the metal content of the solder cream to liquefy and the contacts of the electronic components to be fused and to adhere to the pads on the circuit board. Examples of solder compositions are described in U.S. Pat. No. 3,684,533 to Conwicke and U.S. Pat. Nos. 4,373,974, 4,509,994 and 4,531,986, to Barajas.
In the solder compositions of the above patents, as previously noted, the vehicle for the solder cream generally contains a rosin-type or rosin-base flux. Recent articles by well-known experts, e.g., M. A. Stein, et al, in the November, 1985 issue of "Circuit Manufacturing", point out the advantages of dense solder creams, i.e., an alloy-rich mixture of solder alloy powder and rosin-base flux. Tests conducted by Stein, et al, indicate that a 90% alloy solder cream produces superior solder joints and essentially no solder balls, whereas an 85% alloy cream yields more porous solder joints encircled by halos of minute solder balls which require careful cleaning. The 90% alloy solder cream, however, is too viscous to screen fine lines and is too dry to firmly attach surface-mounted components.
It is accordingly an object of the present invention to provide an improved alloy-enriched solder composition which is especially adapted for soldering components in electronic circuits, particularly densely packed microelectronic circuits.
Another object is the provision of a solder composition of the above type containing a rosin-base flux which forms dense solder joints and without yielding solder balls.
A still further object of the invention is to provide an efficient solder composition of the above type which forms minimal residue.